The operation point of a centrifugal pump can be estimated using a torque estimate (Test) and a rotational speed estimate (nest) from the frequency converter and the QH and QP characteristic curves provided by the pump manufacturer together with affinity laws. This method is referred to as QP calculation. The estimate of the operation point (volumetric flow Qv and head h) obtained with the calculation is most accurate at the nominal (i.e., best efficiency) operation point of the pump, and its accuracy becomes poorer when moving away from the nominal operation point. This limits the usability of the QP calculation in estimating the operation point of the pump. An alternative estimation method or improvement of the existing QP calculation algorithm is, therefore, required for the accurate estimation of the operation point of a centrifugal point, when the pump is operating outside/away from the nominal point.
One reason for the inaccuracy of QP calculation is that the slope of the QP curve gets lower when the efficiency of the pump decreases, which takes place when moving away from the nominal operating point. This causes errors in the estimation of the volumetric flow and head produced by the pump. Another reason for the inaccuracy is the fact that a notable change of the rotational speed can affect the efficiency of the pump. In addition, the amount of mechanical losses in the pump at different speeds may affect the accuracy of the affinity laws. These factors are not typically taken account in the affinity laws.
When the pump operates in a normal manner, the operation point is always situated at the intersection of QH curves of the pump and the process. This is illustrated in FIG. 1, in which an example of a QH curve of a process is drawn against the QH characteristic curve of a pump. The QH curve of the pump is presented in FIG. 1 as a set of curves drawn at different rotational speeds of the pump. The example of FIG. 1 also includes the efficiency of the pump. Thus, it can be read from the curves of FIG. 1 that when the pump is operated at the speed of 1400 rpm, the pump produces a head of 17 m and the output of the pump is 30 l/s. Further it can be seen that the pump is operated at its most efficient operating point, the co-efficient of efficiency being about 73%.